It has hitherto been known that a thermoplastic resin composition is pre-expanded without inducing crosslinking by dispersing thermoplastic resin particles containing a volatile blowing agent in water while stirring in a pressure-resistant container, such as an autoclave, and releasing the dispersion into a lower pressure zone while being kept at a high temperature under a high pressure, as described in West German Patent Application (OLS) No. 2,107,683 and JP-B-56-1344. (The term "JP-B" as used herein means an "examined Japanese patent publication".)
Polystyrene can be obtained in the form of spherical resin particles by suspension polymerization, and pre-expanded particles prepared by using the polystyrene particles have a spherical form. On the other hand, pre-expanded particles of polyolefin resins, e.g., polyethylene and polypropylene, can be obtained by using uncrosslinked resin particles, whereas pre-expanded particles having a spherical form and a uniform cell diameter can hardely be obtained from pellets or pulverized pellets of polyolefin resins and, besides, the resulting pre-expanded particles show scatter of expansion ratio. Such irregular pre-expanded particles cannot be effectively charged in a mold, failing to obtain a uniformly expanded molded article. Therefore, there is room for further improvement in a process for producing pre-expanded particles of polyolefin resins.